Method for stripping layers of organic material

ABSTRACT

Layers of organic material, especially of polymerized photoresist, are removed by means of a mixture of an at least 95% H2SO4, and an at least 30% H2O2, at a ratio of at least 15:1 (referring to the anhydrous chemical substances). The H2O2 content of the aqueous H2O2 is to be in a ratio of at least 11:1 to the H2O content of the H2SO4. The cleaning effect is based on the dehydrating effect of the H2SO4, and the oxidizing effect of the H2O2.

United States Patent Beck et al. Aug. 19, 1975 [54] METHOD FOR STRIPPING LAYERS OF 3,676,219 7/1972 Schroeder et a1 156/13 ORGANIC MATERAL 3,728,154 4/1973 Suzuki 156/17 3,752,714 8/1973 Ito et a1. 156/17 Inventor-S1 g g Beck, ng 3,801,512 4 1974 Solenberger 1. 1561/18 Friedrich C. Brunner, Sindelfingen; Peter U. Frasch, Boeblingen; Blanka Ivancic Boeblingen; Friedrich Primary ExaminerCharles E. Van Horn Schwerdt, Boeblmgefli Theodor Assistant Examiner lerome W. Massie vogtmanna Holzgerlmgen of Attorney, Agent, or Firm-David M. Bunnell Germany [73] Assignee: International Business Machines Corporation, Armonk, NY.

57 ABSTRACT [22] Filed: Apr. 5, 1974 1 [21] App! 458,254 Layers of organic material, especially of polymerized photoresist, are removed by means of a mixture of an 52 us. c1. 134/3; 96/36; 156/17; at least 95% "2 4 and at least 30% 2 2 at 8 252/792 ratio of at least 15:] (referring to the anhydrous 51 1111.0. HOlL 21/312; c091 13/04 ehemieel Substances) The "2 2 eentent ef the q s 1 Field of Search 252 792, 79.1; 156/7, Que "2 2 is to be in a ratio of at least 1111 to the "2 56/13 17 96/36 134/37, 33, 40 content of the H 80 The cleaning effect is based on 3 the dehydrating effect of the H 80 and the oxidizing effect of the H. 0

[56] References Cited UNITED STATES PATENTS 8 Claims, 1 Drawing Figure 3,373,114 3/1968 Grunwald 252/793 METHOD FOR STRIPPING LAYERS OF ORGANIC MATERIAL NEGATIVE RESIST 11115 REQUIRED FOR STRIPPING a z 1 5 P0$ITIVE RESIST llll|llllllllllllli WEIGHT "/1 11 PATENTEDAUGISIQYB 3,900,337

METHOD FOR STRIPPING LAYERS OF ORGANIC MATERIAL NEGATIVE RESIST TIME REQUIRED 15 l FOR STRIPPING vI I0 I z z 5 -POSIT|VE RESIST OIIIIIIIIIIIIIIIIIIII WEIGHT H2805 3 ,9 00,3 3 7 1 2 METHOD FOR STRIPPING LAYERS OF ORGANIC the time required for stripping positive or negative pho- MATERIAL toresist as a function of the weight percent contents of BACKGROUND OF THE INVENTION The invention relates generally to a method for stripping layers of organic material on the surface of substrates which are resistant to oxidizing acids and more particularly to a method for stripping layers of polymerized photoresist.

In the manufacture of very small components by means of chemical and physical methods,'the cleanness of surfaces considerably influences the yield. Because dirt is frequently of organic origin, effective methods are needed for removing organic material from the surface of the components. This problem is of particular importance in the semiconductor field.

The development of semiconductor technology is characterized by growing miniaturization and everincreasing integration. With the dimensions used today in semiconductor components for conductive lines, line spacings, and active components, individual dirt particles and spots with a diameter of s 5p. can cause defects which spoil entire integrated circuits. It is true that semiconductor components are normally made in clean rooms or at least in clean work stations,-but such measures cannot completely prevent dirt particles from reaching semiconductor surfaces. Besides, it is a characteristic feature of semiconductor technology that organic materials, as eg photoresist in photolithographic process steps, have to be applied to the surface of the semiconductor wafer. For that reason, effective methods are needed for removing organic material, particularly photoresist, from semiconductor surfaces. A dry method is known in the art where organic material is burnt in an oxygen glow discharge. The method removes organic material, particularly photoresist, quite reliably but it requires much time and a complex apparatus and it involves the risk of ion generation in oxide layers due to electron bombardment. Such ion generation can present problems particularly in FET components. Liquids are known for stripping positive and neg ative photoresists which include phenol and a sodium salt. Although the cleaning effect of the liquid is satisfactory, it is immiscible with water which complicates its use. Phenols also provide disposal problems. Sulphuric acid heated to 150C has also been used for stripping photoresist, but this method is dangerous for use in a manufacturing process, and besides the acid soon becomes useless owing to discolouring or contamination through material with high carbon contents.

It is also known from the publication Kodak Seminar on Microminiaturization 4/3 and 4/65, Kodak Pub. P-77 (4/66), p. 37, Eastman Kodak Co., to use a mixture consisting of equal parts of H 50 and H 0 in order to strip polymerized photoresist from semiconductor substrates. However, in this publication this mixture and its vapours are described as highly corrosive and dangerous, and the lifetime of the mixture is given as minutes, which means that -it would not be suitable for manufacturing purposes. This mixture does not appear to be fully satisfactory at room temperature in view of the relatively low H SO content and in the final lecture of that seminar it is pointed out that the mixture is used when hot.

DESCRIPTION OF THE DRAWING The drawing is a graph showing the dependence of the mixture of Caro acid.

BRIEF SUMMARY OF THE INVENTION It is the object of the invention to provide a method for the reliable, fast and inexpensive stripping of layers of organic material from substrates. The method is realizable without a complex apparatus, prevents the metal-contamination of the partsto be cleaned, and employs chemical substances that can be disposed of without pollution problems.

According to the process of the invention, this object is achieved by providing a mixture of H and H 0 in a ratio of at least 15:1, refering to volume quantities of the anhydrous chemical substances using at least by weight sulphuric acid (H 80 and an at least 30% by weight aqueous hydrogen peroxide (H 0 The substrates carrying the organic material are first immersed in the mixture, rinsed under running water, and finally dried.

The stripping mixture for use in the process of the invention has a lifetime of more than three weeks and does not contain any metal ions. The mixture is miscible with water in any ratio, so that it can be rinsed quickly and cheaply and disposed of without any problems. The mixture can be used at room temperature, is inexpensive, and has excellent cleaning characteristics.

DETAILED DESCRIPTION 1 It has been found that the cleaning efficiency of the stripping mixture is strongly influenced by the water content of the original components. A low water content in the H SO is needed and the amount of water in the H 80 has a relatively greater effect on the action of the mixture than a high water content in the H 0 It is advisable, therefore, to use amounts of the two components such that the percent by weight H 0 content of the aqueous H 0 solution is of a ratio of at least 11:1 to the percent by weight water content of the H2804. I H i g I For making the mixture it is preferred that the I-I SO. and H 0 are mixed in a ratio between about 17:1 and 35:1 acid to peroxide relative to each other (referring to volume quantities of the anhydrous chemical substances). A mixture containing more H 0 would be unnecessarily expensive and the dehydrating effect of the mixture would be suppressed too strongly. In a mixture containing less H 0 the oxidizing effect is too low. Within the given range it may be advisable, according to the characteristics of the meterial to be removed, to put the stress either on the dehydratingor on the oxidizing effectof the mixture. A ratio of at least about 15:1 acid to peroxide is required.

To accelerate the cleaning process, it is advisable to immerse the substrate with the organic material first for a predetermined period in a mixture with an H 80 content approaching the upper limit of the above cited mixture ratio, and subsequently for a predetermined period in a mixture with an H 80 content approaching the lower limit. In this version of the process the material to be removed is subjected in the first bath to a strongly dehydrating effect, and in the second bath to a strongly oxidizing effect. No compromise between the oxidizing and dehydrating effect of a single mixture has to be made and it is possible to reduce the overall cleaning time. It should also be pointed out that it is not 3 advisable to do without added H50 in the first bath because the oxidizing effect of the bath is required for keeping it clean and for permeating thick layers of organic material.

hydrating and the oxidizing effect are proportional to the H 80 and the H contents in the mixture. Expecially in compounds with many OH-groups, which include e.g. positive photoresists, it is advisable to sup- I Th ixt r n be pl y d qui g n r lly for port the dehydrating effect of the mixture, i.e., to instripping organic materials from substrates which are crease the H 50 percentage in the mixture to the disnot affected by the mixture themselves. It can be used d t e f the B 0 percentage. particularly advantageously for destroying organic ma- The mixture, which is preferably prepared in a polyterial on semiconductor wafers, particularly those with fluorocarbon beaker, is ready for use after having been field effect transistors because it does not contain any cooled to room temperature. The substrates with the metal ions and can be rinsed off completely with H O. organic material are immersed in the mixture for a pre- Because itis possible to strip organic residues which determined period sufficientto remove the organic maexist in relatively large quantities and layer thicknesses, t'e'rial. Subsequently, they are rinsed in a rinsing casthe process is particularly suitable for removing excade of de-ionized water until the incoming and the posed photoresist. i l5 outgoing water shows the same conductivity, and filn making the mixture, concentrated sulphuric acid nally they are dried.

isadded .to an aqueous solution of hydrogen peroxide The mixture has a lifetime of at least three weeks, but (H 0 The following endothermic reaction takes only under the condition that chemical substances of a place: grade of at least DAB6 (Deutsches Arzneibuch Nr.6)

\ are used (reagent grade). It has been found that de- ,l-l SO H O H 80 H O composition starts immediately after mixing if chemical b I substances of the technical grades have been em- The mixture temperature upon the mixing of H 80 ployed. The cleaning effect can be increased still. furammo, supplies the necessary reaction heat. If for inther, or accelerated, respectively, if two baths are used, stance 955 milliliters of 95 to 97% H 80 are" slowly 'one being more dehydrating, i.e. richer in H 80 and addedto 45 miliilit ers of 85% H 0 the temperature of the second more oxidizing, i.e. richer in Caro acid. it the mixture rises to 70C. should be observed in that connection that a first bath The effective components of the mixture are the dewhi h is nly f a dehyd ating effect, which 0011- hydrating H 80, and the oxidizing Caro acid (H 80 sists of pure H 80 is not desirable because dehydrated The organicmaterial is transferred by dehydration into but not completely oxidized particles quickly contamia material of higher carbon content which subsehate the first bath and would thus make it useless, i.e., quently, upon oxidation, is oxidized substantially to the lifetime of the bath will be increased by added form CO and H 0. V v 7 I H 0 besides, the dehydrating and the oxidizing effect Surprisingly it has been found out that 'too much has to be of a cooperating nature for thicker organic W at e r es pecially in the added l -l SO.,"p revents the formlayers in order to completely penetrate the organic maing of.the Caro acid and thus strongly reduces the oxiterial with the mixture. For that reason, it is not advisdizing effectof the mixture. The damaging effect of the able to let the content of Caro acid of the second bath water injthe H 80 canonly be compensat'ed by a drasreach too high a peak, apart from the fact that the mixtic reduction of the water contents in the H 0 Thus, ture would then be more unstable and unnecessarily exi to give an example, mixtures of 95%H SO. and 85% 4O pensive.

H O or 9 7,5% H SO and 30% H O have a removing The following six examples illustrate the process with effect on the organic material, but a mixture of 95% more detail but are not intended to be limiting. H SQ, and 50% H50 has only a low oxidizing effect. In all examples, 2O semiconductor wafers with an ap- The content of Caro acid in the'mixture is calculated proximate diameter of "57 mm have been coated with from the content of active oxygen in the mixture, which photoresist; then they were exposed through a mask, can be determined, e.g. oxidimetrically, by using an arand the exposed image was developed. Finally, the resenite solution and by back titration with Cer-IV- maining zones of polymerized photoresist in the mixsulphate with osmium tetroxideas catalyst and ferroin ture were removed. as indicator. Qualitatively, the effect of the mixturecan The materials, mixtures, and test conditions embe tested by immersing a wad of cotton. If the cotton ployed with characterize the individual examples are shows a change in color the destroying effect of the listed in Table 1 below. mixture l fi fi 'yiv v The polymerized photoresist was removed com- The drawing shows that there is a composition range pletely and without residues from all semiconductor of the mixture where the cleaning effect is particularly S5 wafers. high. This range lies between 3 and 10 percent by Besides, it was found through continuous tests that weight of Caro acid and is established by mixing volper liter ofthe mixture used in Example 6, polymerized ume quantities, refering to anhydrous chemical subphotoresist covering one side of more than 2000 semistances, of H SQ and H 0 with a ratio of 35:1 and conductor'wafers with an approximate diameter of 57 17:1. The maximum of the curve is plausible as the de- 60 mm can be removed completely.

TABLE 1 Bath No. 1 B th No 2 Example H2O2:H2SO4 Immersion H2O2;H2SO4 Immersion H202 H2304 Organic No. By Volume Time (Min.) By Volume Time (Min.) Concentr. Concentr. (Layiiiifidkiress) By Weight By Weight I 1210 7 30% 2 97.5% negative photoresist ()71 3 97.5 7! positive photoresist TABLE 1 Bath No. 1 Bath No. 2 Example H O :H SO Immersion H G- M 80 lmmersion H O H 50 Organic Material No. By Volume Time (Min.) By Volume 'lirne (Min) Concentr. Concentr. (Layer thickness) By Weight By Weight 3 1:10 3 1:6 2 30% a 97.5% negative photoresist 4 1:10 3 1:6 3 30% B 97.5% positive photoresist 5 1:21 3 1:21 3 85% 2 9S negative photoresist 6 1:21 5 l 21 5 85% B 95 positive photoresist providing a mixture of H 80 and H 0 in a ratio of at least about 15:1 acid to peroxide, referring to volume quantities of the anhydrous chemical substances, using at least 95% by weight sulphuric acid and an at least H 0 by weight aqueous H 0 solution; and immersing said substrate in said mix ture.

2. The method of claim 1 including the steps of rinsing said substrate in water and drying said substrate after the resist has been stripped by immersion in said mixture.

3. The method of claim 1 in which the percent by weight H 0 content of the aqueous H 0 solution is in a weight percent ratio of at least 1 1:] relative to the percent by weight water content of the H 4. The method of claim 1 in which the volume quantities of H 80 and H 0 are in a ratio between about 17:1 and 35:1.

5. The method of claim 3 in which the volume quantities of H 80 and H 0 are in a ratio between about 17:1 and 35:1.

6. The method of claim 5 in which said substrate with the organic material is first immersed for a predetermined period of time in a mixture with an H 80 content approaching the upper limit of the mixture ratio, and subsequently for a predetermined period in a mixture with an H SO content approaching the lower limit.

7. The method of claim 1 in which said organic material is removed from a semiconductor wafer.

8. The method of claim 1 in which said organic material is a patterned photoresist. 

1. METHOD FOR STRIPPING LAYERS OF ORGANIC MATERIAL FROM THE SURFACE WHICH IS RESISTANT TO OXIDIZING ACIDS COMPRISING: PROVIDING A MIXTURE OF H2SO4 AND H2O2 IN A RATIO OF AT LEAST ABOUT 15.1, ACID PEROXIDE, REFERRING TO VOLUME QUANTITIES OF THE ANHYDROUS CHEMICAL SUBSTANCES, USING AT LEAST 95% BY WEIGHT SULPHURIC ACID AND AN AT LEAST 30% H2O2 BY WEIGHT AQUEOUS H2O2 SOLUTION AND IMMERSING SAID SUBSTRATE IN SAID MIXTURE.
 2. The method of claim 1 including the steps of rinsing said substrate in water and drying said substrate after the resist has been stripped by immersion in said mixture.
 3. The method of claim 1 in which the percent by weight H2O2 content of the aqueous H2O2 solution is in a weight percent ratio of at least 11:1 relative to the percent by weight water content of the H2SO4.
 4. The method of claim 1 in which the volume quantities of H2SO4 and H2O2 are in a ratio between about 17:1 and 35:1.
 5. The method of claim 3 in which the volume quantities of H2SO4 and H2O2 are in a ratio between about 17:1 and 35:1.
 6. The method of claim 5 in which said substrate with the organic material is first immersed for a predetermined period of time in a mixture with an H2SO4 content approaching the upper limit of the mixture ratio, and subsequently for a predetermined period in a mixture with an H2SO4 content approaching the lower limit.
 7. The method of claim 1 in which said organic material is removed from a semiconductor wafer.
 8. The method of claim 1 in which said organic material is a patterned photoresist. 